Late years, in connection with circumstances such as an increase in non-metallic inclusions (typically, Al2O3) in molten steel along with a trend toward a higher grade of steel, etc. (in the present invention, the terms “non-metallic inclusions”, “Al2O3 inclusions” and “inclusions” are used as substantially synonymous terms), adhesion of inclusions as typified by Al2O3 onto a surface of an inner bore of a continuous casting nozzle, or clogging of the inner bore due to the inclusions, has been becoming one key factor determining a usable life of the continuous casting nozzle.
In the above circumstances, there has been an increasing need for enhancing durability of the continuous casting nozzle by preventing the adhesion of non-metallic inclusions onto the inner bore surface or the clogging of the inner bore due to the non-metallic inclusion. Therefore, with a view to preventing the adhesion of inclusion components (typically, Al2O3) from molten steel onto the inner bore surface, etc., various proposals have been made about a refractory layer on the side of the inner bore surface of the continuous casting nozzle.
For example, the following Patent Document 1 discloses a continuous casting nozzle in which at least an inner bore portion and/or a molten steel-contacting portion thereof comprises an Al2O3—SiO2 based refractory material which contains no carbon component, and has a chemical composition comprising 5 to 10 weight % of SiO2 and 90 to 95 weight % of Al2O3, wherein a dominant mineral phase of the refractory material consists of mullite, and corundum and/or β-Al2O3.
However, such a refractory material containing no carbon component has an extremely low resistance against thermal shock, and thereby involves a particularly high risk of breakage due to thermal shock occurring, for example, at the start of pouring of molten steel. Moreover, despite containing no carbon component, the above Al2O3—SiO2 based refractory material is still incapable of sufficiently preventing the adhesion of inclusions as typified by Al2O3 or the clogging of the inner bore.
Therefore, many proposals have been made which are intended to incorporate a large amount of CaO component capable of reacting with inclusions as typified by Al2O3 to readily produce a low melting point substance, into a raw material for the refractory layer on the side of the inner bore surface, to prevent the adhesion of inclusions or the clogging of the inner bore.
For example, the following Patent Document 2 discloses a technique of arranging a lining layer along an inner bore of a continuous casting nozzle, wherein the lining layer has a composition comprising 40 to 90 weight % of CaO, 0 to 50 weight % of MgO and 0 to 20 weight % of C. However, particularly in cases where a large amount of CaO is contained in the lining layer, it is difficult to use the continuous casting nozzle in an actual continuous casting operation, because CaO exists as free-lime susceptible to hydration, and the hydration causes breakage or the like of the nozzle. Moreover, the composition comprising CaO has significantly large thermal expansibility, and a thermal expansion of the lining layer due to the composition causes breakage of a body layer i.e., a continuous casting nozzle body, on an outer side of the lining layer.
As measures for the problem related to CaO, for example, the following Patent Document 3 discloses a ZrO2—CaO-containing continuous casting nozzle which comprises 16 to 35 weight % of CaO, 20 to 95 weight % of calcium zirconate-based clinker consisting primarily of CaZrO3, and 5 to 50 weight % of graphite, and the following Patent Document 4 discloses a continuous casting nozzle in which an anti-adhesion layer is arranged as an inner bore surface layer (inner bore-side layer), wherein the anti-adhesion layer is made from a mixture added with one or two selected from the group consisting of: 40 to 85 weight % of zirconia clinker (containing Cubic ZrO2 and CaZrO3 as a mineral composition) containing 3 to 35 weight % of CaO; 10 to 30 weight % of graphite; 1 to 15 weight % of silica; and 1 to 15 weight % of magnesia. These materials are designed to allow CaO to exist as a mineral having a crystal structure with ZrO2, etc., in order to prevent CaO from existing as free-lime.
However, the refractory materials comprising the above components has little effect in preventing the adhesion of inclusion components (typically, Al2O3) onto the inner bore surface, etc., in an actual continuous casting operation, and thereby it is impossible to ensure sufficient durability, such as sufficient durable hours (usable life), of the continuous casting nozzle. Moreover, although the problem of hydration can be solved, thermal expansibility of the inner bore-side layer cannot be reduced to a level equal to that of an Al2O3-graphite based refractory material for a conventional continuous casting nozzle body located on an outer side of the inner bore-side layer. Thus, for example, in a structure integrally provided with the two layers, it is impossible to sufficiently prevent breakage of the continuous casting nozzle due to thermal shock.
As a prerequisite to providing such a layer having large thermal expansibility on the side of the inner bore, it is necessary to increase thermal shock resistance in terms of a structure of the continuous casting nozzle. For example, the following Patent Document 5 discloses a casting nozzle in which a base material nozzle is formed on an outer side of a CaO nozzle made of a refractory material comprising 70 weight % or more of CaO and having an apparent porosity of 50% or less, and a gap corresponding to a thermal expansion amount of the CaO nozzle is provided between the CaO nozzle on the side of the inner bore and the base material nozzle on the outer side of the CaO nozzle.
However, as in the Patent Document 5, if a continuous casting nozzle is formed in a special structure having a gap provided between an inner bore-side layer and an outer periphery-side layer, it has a problem, such as difficulty in forming the two layers into a usual integral shaped body for use in a continuous casting operation. Moreover, it has another problem, such as an increase in risk of causing displacement or peeling of the inner bore-side layer, or damage or breakage of the continuous casting nozzle.
Further, the following cited Document 6 discloses a continuous casting nozzle for reducing adhesion of non-metallic inclusions onto the nozzle so as to prevent clogging of the nozzle, wherein the continuous casting nozzle is entirely or partly made of a refractory material having a composition comprising: 20 to 80 weight % of Al2O3; 10 to 45 weight % of graphite; 1 to 20 weight % of SiO2; and 0.1 to less than 3 weight % of CaO or 0.1 to 5 weight % of oxide of a Group IIa element, except Ca, whereby the continuous casting nozzle can be produced at a low cost.
However, in the refractory material having the above composition, a low melting point substance based on a reaction between Al2O3, SiO2 and CaO is simply produced in the entire refractory material containing these components. Specifically, although SiO2 is volatilized to migrate, Al2O3 and CaO form Al2O3—CaO melt in a microstructure without migrating from their initial sites, and the SiO2 component dispersed over the microstructure is absorbed in the melt to further accelerate the formation of a liquid phase, so that the melt is stabilized in the microstructure as CaO—Al2O3—SiO2, which makes it difficult to form a film with a high coverage, on the side of a working surface. Moreover, after start of casting, the refractory components are supplied from Al2O3 and SiO2 aggregates to the melt over time, so that an amount of low melting point substance to be produced is increased, which causes a problem in terms of high-temperature strength and corrosion resistance.
Consequently, in the above composition, a low melting point substance cannot be sufficiently produced between the working surface and Al2O3 inclusions in contact therewith, and thereby the effect of preventing the adhesion of Al2O3 inclusions inevitably becomes significantly limited. Thus, Al2O3 inclusions are likely to adhere onto the working surface over time, within a relatively short period of time. Moreover, Al in molten steel is oxidized by SiO (gas) released from the working surface, which is apt to accelerate formation and adhesion of Al2O3. For this reason, the Al2O3—SiO2—CaO based material having the above composition has failed to become widely used as an adhesion-resistant material.
As above, a continuous casting nozzle having a CaO-based refractory material arranged on the side of an inner bore thereof often involves a difficult problem in terms of structure, production, handling, performance, etc., and there remain many unsolved industrial problems, such as the necessity of considerable labor and cost for overcoming the problem related to CaO.